In certain engines such as small diesels employing four valve direct-acting double-overhead-camshaft cylinder heads, the camshaft journals are located between adjacent cylinders rather than at the transversal centerline of the cylinder as is more common with gasoline engines. The reason for this difference is that in gasoline engines the diameter of the spark plug, the diameter of the spark plug installation tool and the surrounding spark plug well, combined with a relatively long distance between center lines of both camshafts, allow the journals to be placed at the transversal centerline of the cylinder. On the other hand, in a small diesel, one is forced to locate the camshaft journals between adjacent cylinders because the center space between the centralized injector and/or prechamber and injector in the transversal plane is small, the camshaft and camshaft journals are large in diameter, and the camshaft centers are relatively close. As a result, it is necessary to build the structure supporting the camshaft bearings and tappet guides separate from the lower cylinder head to allow the headbolts to be installed and tightened initially before the cam bearing and tappet guide structure is installed and fastened to the lower head. In other words, a two-piece cylinder head is required.
Presently, the only diesel engine in production with this approach is the Mercedes-Benz engine introduced in October of 1993. Some designs proposed by me in some of my other copending patent applications also generally conform to this new category of two-piece cylinder head engines. In each of these designs, the lower portion of the cylinder head incorporates intake and exhaust ports, valves, retainers and locks as well as a centrally located spark plug for gasoline engines or, alternatively, a fuel injector or a prechamber and fuel injector if the engine is a diesel. The cam and tappet carrier or upper head, which is bolted to the lower head and carries the camshafts, tappet guides and inverted-bucket tappets, is covered by a valve cover. The lower head includes passages through which coolant flows while the upper head is bathed in oil.
In the aforementioned Mercedes-Benz diesel engine and also in a Porsche version disclosed in U.S. Pat. No. 5,080,057, the valve cover extends downwardly on all four sides to meet the four walls of the lower head that extend upwardly a small distance over the top deck of the lower head. A gasket is located between the valve cover and the lower head at a point much lower than the transversal horizontal plane intersecting the cam centerlines. In addition, the exhaust ports exit conventionally horizontally on a plane perpendicular to the longitudinal axis of the cylinder to provide double wall and water jacket separation between the hot exhaust gases and the oil in the upper portion of the cylinder head. In both the Mercedes-Benz design and the Porsche design, while the intake ports extend outwardly and upwardly at a angle to the flat deck of the engine block to improve the air flow, the structure of the cylinder head, starting at the short side walls of the lower head portion to a point above the upper line of the intake ports, results in an extremely high cylinder head construction. This then places the desirability of an even steeper intake port in conflict with the requirements for a low over-all engine height to allow for a low vehicular hoodline and reduced weight. In addition, in both of these designs, the intake manifold is bolted to the cylinder head utilizing machined surfaces, drilled and tapped bolt holes, and a plurality of fasteners, creating a situation which increases cost, weight, complexity and engine width to improve manifold design. Furthermore, in the Mercedes-Benz design, the high pressure fuel line feeding the injector must enter through one of the sides of the cylinder head, either through the short sidewall or through the valve cover, requiring the use of rubber grommets to insulate and seal the fuel line. The problem with using rubber grommets in such locations is that they eventually shrink, harden and crack, thereby causing oil leaks.
Thus, it should be apparent from the above that what is needed is a simpler cylinder head design, lighter, shorter, smaller, cheaper, based on simplified machining, reduced number of fasteners, reduced possibility of oil leaks, with improved porting and simplified manifolding with lower weight and bulk, all in one package.